This invention relates to measuring apparatus in general and more particularly to an improved photometer, detector and amplifier arrangement for use in automatic analysis apparatus.
Photometric analytical apparatus which measure quantities such as the absorbance of light by certain fluids and the like are well known in the art. An improved type of such apparatus is that disclosed in copending application of John G. Atwood et al Ser. No. 499,602 filed Aug. 22, 1974. This application has now been abandoned and a continuation application Ser. No. 594,951 filed July 10, 1975 has been filed on the subject matter thereof. In the photometer disclosed therein, reacting serum samples are provided to a flow cell through which a beam of light is passed. At the same time, another portion of the beam which is used as a reference is passed through an additional optical path. For the light output of the beam after passing through the reacting mixture to be of any value in determining absorbance in the mixture, means must be provided for detecting the light level and for amplifying the detected level to a voltage value which is useful. Various means of providing such an output have been developed. In general, each of these comprises a photodetector along with stages of amplification. To obtain the proper type of output, the amplification must also include amplifiers having logarithmic input or feedback in order to provide an output which is proportional to absorbance. Although many such systems have been developed, all suffer from a certain lack of accuracy and/or a low signal-to-noise ratio.
For example, it has been common practice in the prior art that most sensitive and stable photometers for measuring small changes in absorbance of samples, achieve sensitivity and stability through the use of the double beam and optical modulation system in which a beam of spectrally filtered radiation or portions of the beam from a suitable source of selected wavelengths is alternately directed along a path through the sample and a reference path by passing the sample. The beams sample and reference are the combined on a single detector to produce a periodically time-varying signal. In large part this is a requirement due to the limitations of photodetectors previously used. In these prior art systems, the periodic signal is then demodulated to generate a signal in proportion to the difference between absorbance in the sample and reference paths. In the very best examples of this prior art approach to sensitive photometry, the least detectable change in absorbance is about 5 .times. 10.sup..sup.-4 absorbance unit.
In view of the foregoing state of the art, it is the object of the present invention to provide an improved photometer, detector, amplifier arrangement which provides increased accuracy along with improved signal-to-noise ratio.